The present invention relates to cosmetic compositions for application to human skin. Significant forms of the invention are concerned with antiperspirant compositions for application to human skin, especially the axilla. However, the invention can also be applied to other forms of cosmetic composition.
A wide variety of cosmetic compositions for application to human skin make use of a thickened or structured liquid carrier to deliver colour or some other active material to the surface of the skin. A significant example of such cosmetic compositions are antiperspirant compositions which are widely used in order to enable their users to avoid or minimise wet patches on their skin, especially in axillary regions.
Cosmetic compositions have been made in a variety of product forms. One of these is a so-called xe2x80x9cstickxe2x80x9d which is usually a bar of an apparently firm solid material held within a dispensing container and which retains its structural integrity and shape whilst being applied. When a portion of the stick is drawn across the skin surface a film of the stick composition is transferred to the skin surface. Although the stick has the appearance of a solid article capable of retaining its own shape for a period of time, the material usually has a structured liquid phase so that a film of the composition is readily transferred from the stick to another surface upon contact. Examples of cosmetic compositions which are, or can be, marketed in a stick form are lipsticks, lip salves and eyebrow pencils. The stick form has been used in particular for deodorant and antiperspirant compositions where the composition includes a deodorant active material or an antiperspirant active material respectively.
Another possibility is that a stick is a softer solid composition accommodated in a dispensing container which in use extrudes the composition through one or more apertures.
Antiperspirant sticks and other cosmetic compositions can be divided into three categories. Suspension sticks contain a particulate material, notably a particulate antiperspirant active material, suspended in a structured carrier liquid phase. Emulsion sticks normally have a hydrophilic phase forming an emulsion with a second, more hydrophobic, liquid phase. One of the phases contains an active material. Solution sticks typically have the active material dissolved in a structured liquid phase; this phase may be organic solvent or may be a mixture of water and a water-miscible organic solvent. This classification into suspension, emulsion and solution types can be applied to both firm and soft solid compositions.
Besides firm and soft sticks, a number of cosmetic compositions have taken the form of liquids which are formulated to be somewhat viscous and hence pour and flow more slowly than water. One example is antiperspirant compositions in liquid form, such as applied using a roll-on applicator.
There is substantial literature on the structuring or thickening of cosmetic compositions which is frequently accomplished using some form of thickening agent as part of the composition.
Some compositions have a substantial viscosity, which may even make them capable of retaining their own shape for a time, because of transient interactions between molecules of a thickening agent in the liquid.
This is characteristic of compositions which are thickened with polymers. Thickening can be attributed to interactions between polymer molecules.
It is characteristic of such thickened compositions that their viscosity can be achieved or recovered on standing at room temperature. If subjected to shear their viscosity reduces (hence they are described as shear thinning) but the viscosity recovers towards its original value if the composition is subsequently allowed to stand at room temperature.
Compositions which have two phases, i.e. compositions which are an emulsion or a suspension may also have substantial viscosity, even to the extent of being able to sustain their own shape.
Here too it is characteristic of the composition thatxe2x80x94provided the composition is stablexe2x80x94its viscosity will recover spontaneously if it is reduced by subjecting the composition to shear.
Compositions have also been given structure and an enhanced viscosity or rigidity by the incorporation of a structurant (also referred to as a gellant or gelling agent) which causes the liquid to gel upon cooling from an elevated temperature.
Gel formation takes place as an exothermic event within a temperature range referred to as the gel point or gel temperature. Upon reheating, melting of the gel takes place as an endothermic event within a temperature range. When the gel melts, the structurant goes into solution in the liquid. Such gels can be disrupted by shearing and do not recover their viscosity for a long time, if at all unless remelted, although a small partial recovery may be observed.
One material which is well known to form gels is 12-hydroxystearic acid which is discussed in Terech et al xe2x80x9cOrganogels and Aerogels of Racemic and Chiral 12-hydroxy octadecanoic Acidxe2x80x9d, Langmuir Vol 10, 3406-3418, 1994. The material is commercially available from Ajinomoto and from Caschem.
U.S. Pat. No. 5,750,096 and U.S. Pat. No. 5,489,276 are two of several documents which teaches that gelation can be brought about using esters or amides of 12-hydroxystearic acid.
In these documents, especially the latter, there is a proposal to enhance the efficacy of 12-hydroxystearic acid or other gellant by including a xe2x80x9cnucleating agentxe2x80x9d. A range of possibilities for the nucleating agent are suggested, including esterified sucrose. The only function attributed to that material is to modify or promote the gelating action of another material.
Gelation with a waxy material, especially stearyl alcohol, has been mentioned in a great many documents. WO-A-98/43605 is a recent example.
N-acyl amino acid amides and esters are also known to structure liquids. We have established that they do so by forming fibrous networks. They are described in U.S. Pat. No. 3969087. N-Lauroyl-L-glutamic acid di-n-butylamide is commercially available from Ajinomoto under their designation GP-1.
Further materials which have been disclosed as gelling agents are the amide derivatives of di and tribasic carboxylic acids set forth in WO 98/27954 notably alkyl N,Nxe2x80x2dialkyl succinamides.
Yet another example of material which can be used to bring about gelation of a hydrophobic carrier liquid is lanosterol, as described in WO 97/11678 where it is used in the preparation of soft, opaque gels as antiperspirant compositions WO 98/34588 also describes the use of lanosterol as a gellant for oil-based cosmetic compositions.
We have recognised that when a gel is formed, the solution of the structurant in a liquid may supercool before gelling commences, and in consequence the gel-melting temperature may be higher than the gel-formation temperature. If gel-formation takes place in a quiescent solution of the structurant the extent of supercooling may be substantial. We have observed that it varies from one structurant to another.
However, if the structurant is being used to prepare a product where a constituent such as a disperse phase is mixed into the hot liquid before gel formation, we have found that it is likely to be necessary to carry out this mixing operation at a temperature at which the structurant is fully soluble in the liquid. If an attempt is made to mix the composition at a temperature at which there is some supercooling, it is likely that the mixing will induce gelation to commence.
It is a consequence of this that when preparing structured liquid compositions containing a structurant to gel the liquid, all constituents of the composition must be subjected to a temperature high enough to dissolve the structuring agent.
We have now found that advantageous properties and processing can be provided by utilising as gelling agent an esterified saccharide with has an enthalpy of gelation of at least 45 kilojoule per mole.
According to a first aspect of this invention there is provided a cosmetic composition comprising a water-immiscible carrier liquid and a structurant therefor which is effective to gel the composition upon cooling from a temperature at which the structurant is a mobile solution in the carrier liquid, characterised in that the structurant is a wholly or partially esterified saccharide which contains no more than eight monosaccharide residues and which has an enthalpy of gelation in the carrier liquid of at least 45 kJ/mole, preferably at least 48 kJ/mole or 50 kJ/mole.
The enthalpy of gelation can be determined by differential scanning calorimetry (DSC).
We have found that if the enthalpy of gelation is high (45 kJ per mole or more) compositions can be prepared with two advantages. One is that a composition can be prepared using moderate processing temperatures (which we have found to be important). The other is that the resulting gel is stable. By contrast, if the enthalpy of gelation is low, one or both of two disadvantages are observed. Rather high processing temperatures may be needed and/or a gel may be formed but then undergo unwanted progressive transformation during storage, such as crystals appearing and growing, softening of the composition or leakage of liquid from
Although the enthalpy of gelation is a property of the structurant and carrier liquid jointly, we have found that it is predominantly a property of the structurant. Consequently, the measurement of enthalpy of gelation in one or several representative liquids is valuable as technique (which can be carried out with a small sample and a standard instrument) to assess the suitability of a potential structurant. It may be desirable to take a measurement in any one of several representative liquids, since some structurants do not gel all hydrophobic carrier liquids. However, we have found that an 80:20 wt % mixture of decamethyl cyclopentasiloxane and isostearyl alcohol is generally suitable when assessing the suitability of a potential esterified saccharide structurant.
Therefore in a second aspect this invention provides a cosmetic composition comprising a water-immiscible carrier liquid and a structurant therefor which is effective to gel the composition upon cooling from a temperature at which the structurant is a mobile solution in the carrier liquid, characterised in that the structurant is a wholly or partially esterified saccharide which contains no more than eight monosaccharide residues and which structurant is able to gel an 80:20 wt % mixture of decamethyl cyclopentasiloxane and isostearyl alcohol with an enthalpy of gelation of at least 45 kJ/mole, preferably at least 48 or 50 kJ/mole.
The esterified saccharide structurant will generally be such as to exist in the form of a network of fibres or strands within the water-immiscible carrier liquid.
Preferably the structurant contains at least two but no more than five monosaccharide residues and is esterified at a majority of the esterified hydroxy groups.
Preferably the average number of carbon atoms in the esterifying acyl groups is at least 6, more preferably is from 6 to 8 up to 18 carbon atoms.
A composition of this invention may be a structured solution with active ingredient or colourant dissolved in the carrier liquid, or it may be a composition with a dispersed second phase, either a suspension with a solid active ingredient dispersed in the carrier liquid or an emulsion with a second liquid phase dispersed in the carrier liquid. The second liquid phase may then be a solution of an antiperspirant active in a hydrophilic solvent.
A composition according to this invention may take the form of a firm ge-L with apparent rigidity, or a soft solid which is able to retain its own shape for a time (for example if it is taken out of a mould without being subjected to shear) but which is easily deformed by hand pressure. Preferred within this invention are compositions which have sufficient rigidity that they can be regarded as firm solids. The hardness of such compositions can be measured with a penetrometer, in a manner which will be described in greater detail below.
Certain preferred forms of this invention are concerned with compositions which are translucent or transparent. As is already known, translucent or transparent compositions can be obtained if it is possible to match the refractive indices of the different constituent phases present in the composition.
We have found that compositions within this invention which are a novel transparent or translucent emulsion can be obtained by formulating the composition to meet two criteria. Firstly the disperse phase and the continuous phase (consisting of the water-immiscible carrier liquid and the structurant contained within that liquid)should be formulated so that their refractive indices match. The refractive index of the continuous phase will be close to the refractive index of the water-immiscible carrier liquid in it. In order to achieve good light transmission through a composition, the refractive index of the water-immiscible continuous phase and the refractive index of the disperse phase should match within 0.003 units preferably 0.002 units.
Secondly, the matched refractive indices of these two phases should lie in a range which is an approximate match to the refractive index of the structurant. The closeness of match required will depend on the structurant which is used. The refractive index of a structurant can be determined by making trial compositions as explained in more detail below. Such investigation will also show how closely the refractive index of the liquid must be matched to the structurant.
A composition of this invention will generally be marketed in a container by means of which it can be applied at time of use. This container may be of conventional type.
Another aspect of the invention therefore provides a cosmetic composition comprising a dispensing container having at least one aperture for delivery of the contents of the container, means for urging the contents of the container to the said aperture or apertures, and a composition of either of the previous aspects of the invention in the container. Preferred is that a composition of this invention is sufficiently rigid to be accommodated as a stick product in a dispensing container having an open end at which an end portion of the stick of composition is exposed for use.
The compositions of this invention can be produced by processes in which the composition is produced as a mobile liquid at an elevated temperature and allowed to cool to permit gel-formation.
Thus, according to a further aspect of the present invention there is provided a process for the production of an antiperspirant composition comprising, not necessarily in any order, the steps of
incorporating an esterified saccharide structurant into a water-immiscible liquid carrier
optionally mixing the liquid carrier with a disperse solid or liquid phase,
heating the liquid carrier or a mixture containing it to an elevated temperature at which the structurant is soluble in the water-immiscible liquid carrier, suitably followed by
introducing the mixture into a mould which preferably is a dispensing container, and then
cooling or permitting the mixture to cool to a temperature at which it is thickened or solidified.
In this invention it is possible and indeed preferred that the step of mixing with a disperse phase, and any subsequent steps, are carried out at a temperature not exceeding 90xc2x0 C. and possibly not exceeding 85xc2x0 C.
It may be possible to keep below these temperatures throughout the process.
According to a yet further aspect of the present invention, there is provided a method for preventing or reducing perspiration on human skin comprising topically applying to the skin a composition as set forth earlier comprising an antiperspirant active, a water-immiscible liquid carrier and an esterified saccharide structurant therefor.